1. Field of the Invention
The present invention relates to a lens system.
2. Description of the Related Art
Optical systems such as an optical device, an imaging optical system, an observing optical system, a projection optical system, and a signal processing system, in which light or electromagnetic waves are used, and optical apparatuses in which such optical systems are used, have hitherto been known. These optical systems have a drawback that a resolution is limited due to a diffraction which occurs due to wave nature of light or electromagnetic waves.
Therefore, as a technology for realizing an image formation beyond this diffraction limit, using a negative refractive index medium has been mentioned in Literature, “Physical Review Letters, Volume 85, Pages 3966 to 3969, 18 (2000)” by J. B. Pendry, and Literature “Optical Express, Volume 12, No. 20, Pages 4835 to 4840 (2004)” by L. Liu and S. He.
FIG. 7 is a diagram which describes this technology in which, an image formation by a plane-parallel plate 380 formed of a negative refractive index medium 301, is shown. In FIG. 7    t0 . . . a distance between an object point and a left-side surface of the plane-parallel plate 380    t0′ . . . a distance between the object point and a right-side surface of the plane-parallel plate 380    t . . . a thickness of the plane-parallel plate 380    i . . . an angle of incidence    r . . . an angle of refraction    ns . . . a refractive index of the negative refractive index medium 301 with respect to vacuum.
A refractive index of a medium around the plane-parallel plate 380 with respect to vacuum is n0, and in a case of the vacuum, n0=1. FIG. 7 shows a case when n0=1, and ns=−1.
An arrow shows a propagating light emitted by an object. According to the Non-patent Literature “Physical Review Letters, Volume 85, Pages 3966 to 3969, 18 (2000)” by J. B. Pendry, since the law of refraction holds true,n0 sin i=ns sin r  expression 101and when n0=1 and ns=−1, thenr=−i  expression 102.Consequently, propagating light component is focused to an image point where the following expression 103 is satisfied.t0+t0′=t  expression 103
On the other hand, evanescent waves emitted by the object point are restored at a point, where t0′ satisfies the expression 103 and have the same intensity as the intensity at the object point. Since both the propagating and the evanescent light emitted by the object are focused at the image point, an image formation beyond the diffraction limit is realized. This is called as perfect imaging. It has hitherto been known from the Non-patent Literature “Physical Review Letters, Volume 85, Pages 3966 to 3969, 18 (2000)” by J. B. Pendry, that the perfect imaging is realized when expression 103 and the following expression 104 hold true, even when an area around the negative refractive index medium 301 is not a vacuum.ns=−n0  expression 104
In this patent application, a term “light” also includes electromagnetic waves such as microwaves and terahertz waves.
On the other hand, when the image is formed, a single lens inevitably involves a curve of an image plane. Therefore, when a lens 601 having a curved surface as in FIG. 8 is made of the negative refractive index medium 301, a curvature of field occurs. As it is evident from FIG. 8, a principal ray 604 other than an axis passing through a center of an aperture stop 602 forms an image on an image plane 605 having a curvature different from a Gauss plane 603.